This invention relates generally to a thermostat and, more particularly, to an electronic thermostat for controlling multiple temperature altering devices.
Thermostats are well known which sense temperature, compare the temperature to a preselected temperature reference and then produce an actuation signal in accordance with the comparison to control the application of power to temperature altering devices.
Such electronic thermostats require a source of D.C. power. Since D.C. power is not always available directly, such units generally include a D.C. power supply which produces D.C. power from A.C. power. In many commercial applications, the A.C. power is easily connected to the input of the D.C. power supply by one pair of leads while the output terminals and a reference terminal are connected to control relay coils of the temperature altering apparatus being controlled.
Often such thermostats have a heat control terminal and a cool control terminal and a reference terminal. The heat control terminal is connected with a relay control coil associated with a furnace or other heating apparatus. Likewise, the cool control terminal is connected through another lead to a control relay coil associated with an air conditioning unit or other cooling device.
Unfortunately, in many applications which originally employed mechanical rather than electronic switching thermostats, separate A.C. power leads are not available. In such case, electronic thermostat which require separate A.C. power leads for generation cannot be substituted for the mechanical thermostats without expensive new wiring being installed to provide power. As noted in U.S. Pat. No. 4,091,993 of Knecht et al., one of many attempts to solve this problem in two wire systems has been to employ a separate source of power, such as shown in U.S. Pat. Nos. 4,308,991 of Peinetti et al. and 4,373,664 Barker et al. In Knecht et al. power is always drawn through a single control terminal regardless of whether it is actuated or not. In another U.S. Pat. No. 3,815,815 of Sapir, A.C. power is concurrently drawn through both of two control terminals.
Practical considerations also require that the thermostat not cause the temperature altering devices to be short cycled, i.e., to be rapidly turned on and off, any more than necessary, since such short cycling causes damage to electric motors and the like and increases the wear rate on other parts. For like reasons, it is desired to prevent reapplication of power to such devices immediately after temporary power loss when power voltages may still be below rated voltages for such devices.